In conventional pyrolysis processes which use pyrolysis furnaces, reaction mixtures of feed hydrocarbons and steam flow through a plurality of long coils or tubes which are heated by combustion gases to produce ethylene and other olefins, diolefins and aromatic hydrocarbons. The combustion gases are formed by burning hydrocarbon fuel such as natural gas or fuel oil. The combustion gases, which are external to the coils, are passed around the coils, countercurrent to the hydrocarbon feedstock which flows through the coils. Heat is transferred from the hot combustion gases through the walls of the coils to heat the hydrocarbon feedstock passing through the coils. Typically, the hydrocarbon feedstock is heated to about 750.degree. C. to 950.degree. C. However, in recent years, there has been a trend to heat the hydrocarbon feedstock to a higher temperature in order to obtain increased amounts of ethylene production for a given amount of feed. In other words, higher temperature is used to achieve greater selectivity for ethylene production.
The use of higher operating temperatures tends to promote or increase the production and collection of coke, already a known pyrolysis reaction by-product which collects on the inner walls of the coils. Coke is a semi-pure carbon which generally results from a combination of a homogeneous thermal reaction in the gas phase and a heterogeneous thermal reaction between the hydrocarbons in the gas phase and the metal walls of the coils.
Deposition of coke within the coils of a conventional pyrolysis furnace is associated with several deleterious effects. For example:
A. Coke formation on the inner walls of the coils impedes heat transfer to the reaction mixture in the coils or tubes. Thus, a smaller fraction of the heat of combustion is transferred to the hydrocarbon feed and a larger fraction of the combustion gas heat is thereby lost to the surroundings in the stack gas.
B. Due to the increased resistance to heat transfer, the temperature of the walls of the coils must be heated to even higher temperatures to adequately heat the hydrocarbon feed within the coils. This results in increased fuel consumption and damage to the coil walls and produces a shorter life for the expensive high-alloy coils. Typically this heat induced damage is caused by increased corrosion or erosion of the coil walls.
C. The coke build-up in the coils restricts the flow path in the coils and results in a larger pressure drop in the hydrocarbon-steam mixture flowing through the coils. Consequently, more energy is required to compress the hydrocarbon product stream in the downstream portion of the process.
D. The coke build-up in the coil also restricts the volume of reaction mixtures in the reaction zone, thereby decreasing the yield of ethylene and other valuable byproducts and increasing the yield of undesirable by-products (e.g. methane, tar, etc.). Thus the selectivity of the pyrolysis (the ability to produce the target compound for a given amount of feed) is decreased. Consequently, more hydrocarbon feedstock is needed to produce the required amount of the desired target compound.
Coke deposition is also a problem in heat exchangers or transfer line exchangers (often referred to as TLX's, TLE's, or quench coolers). Such coking is typically called "catalytic coking." The objective of a heat exchanger or TLX is to recover as much of the heat as possible from the hot product stream leaving the pyrolysis furnace. This product stream contains steam, unreacted hydrocarbons, the desired pyrolysis products and by-products. High pressure steam is produced as a valuable by-product in the TLX and the product mixture is cooled appreciably. As in the coil of the pyrolysis furnace, coke deposits in a heat exchanger, results in poorer heat transfer which in turn results in decreased production of high-pressure steam. Coke formation in a heat exchanger also results in a larger pressure drop for the product stream.
In addition to the above well recognized locations of coke deposit, coke might form on connecting conduits and other metal surfaces which are exposed to hydrocarbons at high temperatures. A more subtle effect of coke formation occurs when coke enters the furnace tube alloy in the form of a solid solution. The carbon then reacts with the chromium in the alloy and chromium carbide precipitates. This phenomenon, known as carburization, causes the alloy to lose its original oxidation resistance, thereby becoming susceptible to chemical attack. The mechanical properties of the tube are also adversely affected. Carburization may also occur with respect to iron and nickel in the alloys.
In pyrolysis devices currently used, coke formation and accumulation in the pyrolysis coils and/or in the heat exchangers or transfer line exchangers eventually becomes so great that cleaning is necessary. Known cleaning techniques typically require shutting down the pyrolysis unit (i.e. the hydrocarbon feed-stream flows are suspended) during the cleaning or decoking procedure. The flow of steam, however, is generally continued during the decoking or cleaning procedure because steam reacts slowly with the deposited coke to form carbon oxides and hydrogen. Moreover, air is often admixed with the steam. At the high temperatures in the coils, the coke reacts quite rapidly with the oxygen in the air to form carbon oxides. After some time, typically 1-3 days, the coke will generally be almost completely removed by this procedure. This cleaning is frequently referred to as "decoking."
Coke in heat exchangers is not as easily removed or gasified, however, due to the lower temperatures in heat exchangers as compared to the temperatures in the coils. Cleaning or decoking of heat exchangers is therefore often accomplished by mechanical means. Mechanical decoking may also be used for cleaning the coils.
One disadvantage associated with decoking with the steam-air mixture is an insufficient ability to control the combustion temperature in the coil during decoking. The temperature cannot be easily controlled due to the exothermic reactions involved when steam and oxidant react with the coke. Consequently, when a large amount of coke is deposited in the coils, local overheating may occur during decoking. Such overheating may heat the metal coil to a temperature which exceeds the temperature limit of the tube or coil metal. Generally, overheating leads to the development of splits, deformations, and other types of breakage of the tubes especially in the vicinity of the welding stitch.
In 1996, almost 50 billion pounds of ethylene were produced in the United States, primarily by the above-described process. It is anticipated that this production will increase to about 52 billion tons by 1999. In the Pacific Rim countries, about 10 billion pounds of ethylene were produced in 1996, primarily by the above-described process. It is anticipated that production will increase to about 40 billion tons by the year 2000.
For conventional pyrolysis units, decoking must be performed approximately every 30 to 60 days, depending on the hydrocarbon feed quality and the severity of the pyrolysis conditions. In the more modem pyrolysis furnaces such as the "Millisecond" sold by the Kellogg Chemical Company, decokings are performed every 10 days. As noted earlier, decoking generally requires about one to three days, resulting in downtime frequently causing a several percentage loss of ethylene production during the course of a year. Decoking is also relatively expensive and requires appreciable labor and energy. There is a strong incentive to extend the interval between decoking operations.
Numerous methods have been suggested for eliminating or minimizing coke deposition, in hopes of making long-term, continuous thermal cracking of hydrocarbons possible. For example, improved control of the operating conditions and improvement of feedstock quality has resulted in small decreases in the rate of coke deposition. The cost of making such changes, however, is often so high that these changes are frequently not cost effective.
Several processes have been reported in which various additives, asserted as being either inhibitors or catalysts, are added to the hydrocarbon-steam feedstream. If the additives are inhibitors, coke formation is inhibited or minimized. If the additives are catalysts, reactions between the coke and steam are presumably promoted or catalyzed. In such an instance, the formation of carbon oxides (CO and CO.sub.2) and hydrogen are promoted. In either case, the net rate of coke which collects on the metal surfaces is decreased.
For example, sulphur has been proposed for use as an additive to reduce coke depositions in several patents. At least part of the beneficial effect of sulphur is generally considered to be caused by conversion of metal oxides on the inner surfaces of the coil walls to metal sulfides. The metal sulfides tend to destroy the catalytic effect of metal oxides which promote coke formation. Although sulphur may act as an inhibitor, it also frequently promotes the destruction of the coil metal walls by replacing the metal's corrosion resistant, protective oxide layer with metal sulfides which tend to flake off or be lost from the surface. Moreover, at high temperatures, some sulfides such as nickel sulphide, liquify. In addition, it is necessary to remove such inhibitors from the pyrolysis product to avoid contamination of the product by the inhibitors.
None of the approaches taught by the prior art which teach injection of antifoulant solutions to suppress the catalytically carbonizing action of nickel and iron in heat-resistant alloys has been entirely satisfactory. An added problem has been that apparatuses for injecting such solutions into the gaseous stream have not been effective in providing cover of the antifoulant over the entire inner surfaces of the coil or furnace tube.
As generally known, a solution may be dispersed into a process gas stream under several conditions. However, a disadvantage arises when the solution is injected as a continuous stream, because the solution may reside on a support surface as a statistical film, only one side of which contacts the process gas stream. Subsequent evaporation of the liquid, leaves solid residues (additives or solute) deposited on the support surface so that a substantial part of these additives are inactive. Furthermore, the deposition of the solution and subsequent evaporation may result in damage to the coil or furnace tube.
Most methods previously employed for removing coke deposits (i.e., with the steam-air mixture) require that the normal function of the furnace and the coils for cracking hydrocarbon materials be interrupted during the cleaning or coke removal operation. Such interruptions of on-stream time of the pyrolysis furnace produces serious economic problems in view of the unit off-stream operation which is required for the removal of coke deposits and the necessity to return the furnace to on-stream operation after decoking. As described above, normal decoking of the coils or furnace tubes often requires a feed outage of 1-3 days or even longer before decoking is complete. In addition, cycling the furnaces between the on-stream and off-stream mode of operation increases the wear of the tube supports.
Similarly, none of the prior art methods which use additives to remove coke deposits from the inner walls of the coils or tubes in a pyrolysis furnace have been entirely satisfactory. Accordingly, a process would be highly desirable if it could effectively decoke the coils or tubes within a pyrolysis furnace and suppress further coking without having to raise the temperature within the coils during the decoking process and without having to shut down the hydrocarbon feed of all of the coils or tubes within the furnace when decoking one or some of the coils or tubes.